Voyager 2, a portrait before the farewell
How old were you in the summer of 1977? Old enough to remember something?
If yes, try to go back with your memory to that period.
To help you, we will tell you that in those days the first Star Wars was shown in theaters and that
on August 15 of that year the Ohio State University's Big Ear radio telescope received
the famous WOW signal from Space. Do you remember? And that the next day, August 16, in Memphis,
Elvis Presley died (perhaps you remember this more, don't you?).
Well, just four days after the death of Elvis, on August 20...from the
Cape Canaveral base, the Voyager 2 mission left! 43 years have passed, a lifetime...and yet since
then Voyager has still been traveling, and still sending signals from ever more remote distances.
It is the oldest of all of those still operational,
but now perhaps it is time to say goodbye. Let's see why retracing its entire history.
Pasadena, USA, summer 1964 at the Jet Propulsion Laboratory.
In view of future space missions conducted with automatic probes,
the young aerospace engineer Gary Flandro (1934) is assigned the task of studying
new trajectories for the exploration of the external solar system.
And Flandro does his job very well, so much so that he discovered that in a few years
there would be an alignment between the outer planets that would allow a probe launched towards
Jupiter to fly over Saturn, Uranus, and Neptune, only using the "gravitational slingshot" effect
of each planet to reach the next; thus reducing from 40 to 10 years the time to
complete the "Grand Tour", as it was then called the tour of the outer solar system.
But it must be done soon... that particular alignment occurs in fact only every 175 years,
and to take full advantage of the favorable moment the probe must be launched
in a period between 1976 and 1980. Flandro's study, published in 1965,
makes a lot of sensation in the environment, and in 1969 it originates a first NASA mission
proposal called Planetary Grand Tour, later canceled for budgetary reasons;
and then the one for the launch of two twin probes generically called Mariner 11 and 12 (only in 1977
the spacecraft would have received the name of Voyager 1 and Voyager 2), the first one directed
to Jupiter and Saturn and the second one charged to realize the Grand Tour desired by Flandro,
that is: Jupiter, Saturn, Uranus, and Neptune! In the same year, the JPL proposed the
construction of the probes, following the design phase begun in 1966, but it will not
be easy ... In those years there were still many uncertainties about the obstacles that a
spacecraft could encounter in crossing a region of the solar system that had not yet been explored.
Some scientists, for example, believed that the asteroids of the main belt were so numerous
that they prevented a spacecraft from crossing unharmed. Of great help in understanding that the
danger was there, but much overestimated, were the Pioneer probes 10 and 11, which launched
respectively on March 2, 1972, and April 5, 1973, would have been the forerunner for the Voyager.
To make a long story short, after eight years of changes to the scientific and engineering load,
Voyager 2 was launched from Cape Canaveral at 14:29 (universal
time) on Saturday, August 20, 1977. About two years later, on July 9,
1979, the probe reaches the minimum distance from Jupiter,
flies over the planet, and is diverted towards Saturn; receiving also a "push" (thanks to the
slingshot effect) that increases its speed to reach Saturn more quickly.
So Voyager 2 reaches the minimum distance from Saturn on August 26th, 1981, and from
there it is diverted and accelerated towards Uranus, which flies over on January 24th, 1986;
and, finally, from Uranus, it is directed towards Neptune, the remote planet that the
probe reaches on August 25th, 1989. Carried out the "primary mission",
namely that of the Grand Tour of the four gas giants, the glorious spacecraft arrived
at over 30 astronomical units of distance (equivalent to 4.5 billion kilometers)
is assigned the "Voyager Interstellar Mission". The purpose? To extend the exploration of the
solar system to the outer limits of the magnetic sphere of influence of the Sun,
and possibly beyond. And by "influence of the Sun", in this case, we must mean the one exerted
by the solar wind inside the approximately spherical "bubble" (called heliosphere)
that like a cocoon surrounds the planets up to a distance of more than 100 Astronomical Units,
that is, until the force of the radiation that pushes outwards is stronger or equal to that
of the interstellar wind coming from outside. And at that boundary, called the heliopause,
the Voyager 2 arrived already in 2018. And the fact that it did so six years after Voyager 1
(launched on September 5, 1977, but on a different trajectory) is scientifically very
relevant because the two different trajectories will allow evaluating the size of the heliosphere
concerning the eleven-year cycle of solar activity, during which our star reaches
both a maximum and minimum level of activity. However, it should be noted that the heliopause
is only the extreme limit of the heliosphere and not that of the Solar System, as is often
said by mistake. The Solar System is a physical whole dominated by the gravity of the Sun, which
extends its influence beyond the Kuiper Belt and up to the Oort Cloud, about two light-years away
(120,000 astronomical units!). It will take about 300 years for Voyager 2 to reach the inner edge
of the Cloud and probably not less than 30,000 years to overcome it and reach its outer edge...
It can be said then that the two probes (both Voyager 2 and Voyager 1), have just put their
noses outside the door of the house, even if you can already feel the signs of important and
interesting changes. Just these days the data coming from Voyager 2 are confirming
a strangeness already suggested years ago by the sister probe, namely that near the heliopause,
towards the outside, the density of the interstellar medium is not decreasing as it would
be logical to expect, but is even increasing! The average density of the interstellar medium
inside the Milky Way is about 0.037 particles per cubic centimeter. The first measurement made by
Voyager 1 after passing the heliopause, made on October 23rd, 2013 at a distance of 18.3 billion
kilometers, had instead recorded a density of 0.055 electrons per cubic centimeter, thus being
50% higher. A second measurement made when the probe had moved away 20 more astronomical units,
had reported a density of 0.13 electrons per cubic centimeter, even 35 times higher than the average!
Well, also Voyager 2 has recently reported a similar increase in the
density of the interstellar medium once crossed the heliopause. In January 2019, when it was now
124,2 Astronomical Units far away, it measured a density of 0,039 electrons, while six months
later, that value had increased to 0,12 electrons, thus confirming the twin probe detections!
Astronomers are still wondering about the cause of this unexpected radiation increase,
and one of the most credited hypothesis claims that the interstellar magnetic field lines
could intensify near the heliopause, while another one suggests that the material blown
by the interstellar wind could undergo a slowdown in reaching the heliopause,
causing a sort of traffic jam, with an unusual accumulation of matter.
To solve the mystery, other measures will be needed, but at this point,
it becomes impossible not to ask the question: how much longer can the Voyager machines last?
And in fact, the situation does not look rosy. The main problem is energy. The probes travel thanks
to three Radioisotope Thermoelectric Generators that produce energy using the heat emitted by the
natural decay of plutonium-238 radioisotopes; the same type of generators also used, among others,
in the Viking, Pioneer, Cassini and New Horizons missions, but also during the Apollo 12 mission.
The decay of Plutonium atoms guarantees a huge autonomy, but also the plutonium is consumed
over time. It is estimated that the probes have produced about 4 Watts of less power each year,
and to date, therefore, the generators provide about 40% less power than they produced at launch.
The energy is used to operate the on-board systems and scientific instruments, but not only that. It
is also used to "keep warm" the instrumentation. Recently the mission engineers, after careful
discussions and in agreement with the scientific team, have decided to intervene.
To maintain the activity of the probes as long as possible, it is necessary to rationalize the use
of energy ... and therefore it becomes strategic to choose carefully what to keep working and what
to cut instead. Voyager 2 took precedence in this plan, having one more instrument and therefore
also consuming more energy over the years. Among the first actions already put into practice
is to turn off the ultraviolet spectrometer and, recently, a heater of the instrument
for the detection of cosmic rays, but not the energy to make it work. The instrument plays a
crucial role in detecting the number of cosmic rays present in interstellar space. Information
of vital importance for future human space exploration missions. The problem of radiation
is the main problem for long-term missions such as those that humanity is about to undertake.
What just can not risk being left at the mercy of the cold is for obvious reasons
the targeting system, which allows the probes to orient themselves in the right direction
to send data to Earth. If there is no power supply or if the cold weather damages it,
we can no longer receive data or even communicate with the probes to send new instructions.
The challenge then is to figure out what to take energy away from, which of the two probes,
which information will prove to be more important, and which ones are expendable, to continue to do
science as long as possible with both. And it has been calculated that if everything goes well,
the two probes should continue to send signals at least until 2025.
And then? And then the Voyager will remain mute
while continuing by inertia to travel further and further away.
Will they reach other stars? In the very distant future,
it will certainly be possible. What we know now is that 5 million years will pass before Voyager
2 can approach a star less than one light-year, while it will have a less close encounter (2
light-years) in about 42,000 years with the star Ross 248, in the constellation of Andromeda.
Voyagers are destined, perhaps forever, to wander the Milky Way
But, they will never leave our Galaxy. To get out of it and escape its immense gravitational
force they should have a speed of at least 550 km/s when instead their current speed is about
17 km/s! But they will become a satellite, until in millions of years they will be
consumed and pulverized by cosmic rays. The history of the two probes has influenced
not only entire generations of scientists and engineers but also the Earth's culture itself
in its most varied forms, including cinema, music, literature, and art in general.
Each of the probes, however, is already a bearer of culture, carrying a record,
the Golden Record, which conveys sounds, images, and messages from planet Earth. And
since Voyagers are the most distant ambassadors of mankind in space, their journey could last
millions of years and their presence could at some point be the only witness of our existence.
For this reason, the very important goal reached now also by Voyager 2 is not the end of a mission,
nor the end of a journey. It is only the beginning of a new chapter of human exploration.